SUMMARY Phthalates, a class of endocrine disrupting compounds (EDCs) used in plastics and personal care products, are ubiquitous environmental contaminants resulting in widespread human exposure. Epidemiologic data implicate paternal phthalates with adverse reproductive health including poor sperm quality, and more recently, with longer time to pregnancy ? the latter suggests a semen-derived effect. Traditionally, sperm have been considered vehicles only for the delivery of the paternal genome upon fertilization. However, compelling animal data demonstrate that environmental conditions in adulthood are embodied within sperm without altering the underlying DNA sequence, and in turn, these alterations affect offspring health and development. A biological pathway for this paternal contribution to reproductive success has been shown to include aberrations in sperm DNA methylation. To date, research in this area has been largely restricted to the effects of nutritional manipulations in animal models; therefore, relatively little is known about how adult exposures to toxicants affect epigenetic information in sperm ? especially in human populations. We propose that a key to understanding how adult exposures to phthalates impact reproductive health lies within sperm DNA methylation. This application capitalizes on the concurrent sample collection from the Sperm Environmental Epigenetics and Development Study (SEEDS; PI: Richard Pilsner) and the Environmental and Reproductive Health Study (EARTH; PI Russ Hauser), two independent epidemiologic studies investigating the link between paternal phthalate exposure and adverse reproductive health among in vitro fertilization (IVF) populations. Moreover, to address potential generalizability issues stemming from IVF cohorts, we also include in our replication aim, sperm methylation analyses from samples collected from the Longitudinal Investigation of Fertility and the Environment (LIFE; PI Germaine Buck Louis) study, a prospective cohort of couples representing the general population. For our first aim, genome-wide methylation will be analyzed via Illumina?s EPIC array on isolated sperm remaining after in-vitro fertilization (IVF) from SEEDS. Next, we will determine the relationships of sperm methylome and on the reproductive outcomes of embryo quality and probability of a live birth in SEEDS. Finally, after completion of these objectives in SEEDS, we will perform replication studies in sperm samples from EARTH and LIFE to determine the concordance of findings across the three cohorts. The proposed research is expected to uncover pathways linking paternal phthalate exposures with adverse reproductive outcomes via sperm DNA methylation. Characterization of potential intermediate pathways between the exposure and outcome continuum is of significant importance because it will inform avenues of translational research for the development of novel approaches to treat and prevent adverse reproductive health.